Trade winds

A type of easterly wind that exists constantly in the tropics. It is also called the tropical easterly wind. Near the surface of the earth, it blows from the subtropical high toward the equatorial low pressure zone, and is called the northeasterly trade wind in the Northern Hemisphere and the southeasterly trade wind in the Southern Hemisphere. It is particularly prominent over the ocean. The trade wind system forms a large-scale convection current that rises near the equator and descends within the subtropical high pressure zone in a vertical plane running north to south. This is called the trade wind circulation, or the Hadley circulation, named after the British physicist Hadley, who clarified the nature of this circulation. The trade wind layer is thick and reaches a depth of 8 to 10 kilometers or more. The trade wind undercurrent near the surface of the earth (the name trade winds is often used to refer only to this lower level wind) is relatively cold and moist, and above it there is a hot and dry trade wind upper current, and at the boundary between them at a height of 2 to 3 kilometers there is a temperature inversion layer called the trade wind inversion. This structure is particularly noticeable in the southeastern part of the subtropical high pressure system. Cumulus clouds that form over the ocean where the trade winds blow are held down by this inversion layer, and form a unique landscape in which they cluster together in a relatively regular pattern of roughly the same size. These are called trade wind cumulus clouds. The name "trade wind" is a translation of the English word "trade wind," but the original meaning of "trade" is "passageway," and it is believed that the name was coined to refer to winds that blew from a roughly constant direction, making ocean routes possible in the age of sailing ships.

[Atsushi Kurashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

熱帯に定常的に存在する偏東風。熱帯東風ともいう。地表面付近では亜熱帯高気圧から赤道低圧帯に向かって吹き込み、北半球では北東貿易風、南半球では南東貿易風となっており、とくに海洋上で顕著に卓越する。貿易風系は南北方向の鉛直面で、赤道近傍で上昇し亜熱帯高気圧圏内で下降する大規模な対流を形成しており、これを貿易風循環、またはこの循環の本質を明らかにしたイギリスのハドレーの名をとってハドレー循環という。貿易風の層は厚く8～10キロメートルまたはそれ以上に及ぶ。地表面近くの貿易風下層流（貿易風の呼び名を、この下層の風にだけ特定して用いることも多い）は比較的冷たく湿潤で、その上に高温乾燥の貿易風上層流が存在し、その境界の2～3キロメートルの高さに貿易風逆転とよばれる気温の逆転層がある。この構造はとくに亜熱帯高気圧の南東部で顕著である。貿易風の吹く海上で発生する積雲は、この逆転層に上限を抑えられ、ほぼ同じ大きさで比較的規則正しく群立して、独特の景観を形成する。これを貿易風積雲という。なお、貿易風という呼び名は、英語のtrade windの訳語であるが、tradeの原義は「通り道」であり、帆船時代に海上航路を可能にする、ほぼ一定の方向から吹く風の意味で、そのようによばれたものとされている。

［倉嶋　厚］

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